1. Field of the Invention
The present invention relates to an optically active tetrahydropyran derivative, a liquid crystal composition containing the derivative, and a liquid crystal device containing the composition. More particularly, the present invention relates to a novel optically active tetrahydropyran derivative useful as a liquid crystal material used for a display device or an electro-optical device, a liquid crystal composition containing the derivative, and a liquid crystal device containing the composition.
2. Description of the Related Arts
In recent years, fields for application of liquid crystals, such as various kinds of display devices, electronic optical devices, liquid crystal sensors, and the like, have been expanding remarkably, and liquid crystal compounds having various structures have been proposed in parallel with this expansion of the field. Particularly, nematic liquid crystals are mainly used at present as the liquid crystal material for display devices. The nematic liquid crystals are used in a TN type or STN type simple matrix system and a TFT type active matrix system in which a thin film transistor is provided to each picture element. However, driving force of the nematic liquid crystal is based on a weak interaction between anisotropy of a dielectric constant of a liquid crystal material and an electric field. Therefore, the nematic liquid crystal has a drawback in that the response time is essentially low (of the order of msec). Thus, the nematic liquid crystal is disadvantageous as a material for a display device of a large area to which the high speed response is required.
In contrast, a ferroelectric liquid crystal which was first synthesized by R. B. Meyer et al. in 1975 has a spontaneous polarization, and this liquid crystal has a large driving force because this liquid crystal interacts directly with an electric field. Since N. A. Clark et al. reported in 1980 that a surface stabilized ferroelectric liquid crystal device (SSFLCD) has a high speed response of the order of micro-second and a memory effect, the ferroelectric liquid crystal has been attracting attention, and many ferroelectric liquid crystal compounds have been synthesized.
It is well known that the response time of a ferroelectric liquid crystal is expressed by the equation: .tau.=.eta./(Ps.multidot.E). Herein, .eta. represents rotational viscosity, Ps represents spontaneous polarization, and E represents intensity of an electric field. Based on this equation, a liquid crystal material having a lower viscosity and a larger spontaneous polarization has been the target of the development to achieve a high speed response. As a material for a liquid crystal, properties such as chemical stability and a wide working temperature range are required. However, it is difficult to satisfy all the requirements with a single compound. Accordingly, a method of mixing several types of compound having a chiral smectic C phase (SmC* phase), or a method of adding an optically active compound to an achiral host liquid crystal having a smectic C phase (SmC phase) of a low viscosity, was adopted to obtain a ferroelectric liquid crystal composition having desired characteristics and exhibiting the SmC* phase.
In the latter method, the chiral dopant to be added may have or need not have a SmC* phase by itself, but it is required that the chiral dopant has a good compatibility with the achiral host liquid crystal, induces a high magnitude of spontaneous polarization, and does not cause increase in the viscosity.
The spontaneous polarization is considered to arise as the result of restriction of the free rotation of a dipole moment perpendicular to the long axis of the molecule around the long axis of the molecule by the effect of an asymmetric carbon. Accordingly, many attempts to increase spontaneous polarization have been made by such methods as (1) placing a dipole portion at a position close to a skeleton portion which is a so-called core, (2) placing a dipole portion and an asymmetric carbon at positions close to each other, and (3) attaching a sterically large substituent to an asymmetric carbon, and thereby restricting the free rotation around the molecular axis. Further, it has recently been reported that a compound having a structure in which a dipole portion and an asymmetric carbon are directly bonded to a five-membered lactone shows effective restriction of the free rotation and has a large spontaneous polarization (Japanese Journal of Applied Physics, Volume 29, No. 6, ppL 981 to L 983).